Light emitting diode (LED) using three-dimensional gallium nitride (GaN) pillar structures with planar surfaces

ABSTRACT

A method is provided for fabricating a light emitting diode (LED) using three-dimensional gallium nitride (GaN) pillar structures with planar surfaces. The method forms a plurality of GaN pillar structures, each with an n-doped GaN (n-GaN) pillar and planar sidewalls perpendicular to the c-plane, formed in either an m-plane or a-plane family. A multiple quantum well (MQW) layer is formed overlying the n-GaN pillar sidewalls, and a layer of p-doped GaN (p-GaN) is formed overlying the MQW layer. The plurality of GaN pillar structures are deposited on a first substrate, with the n-doped GaN pillar sidewalls aligned parallel to a top surface of the first substrate. A first end of each GaN pillar structure is connected to a first metal layer. The second end of each GaN pillar structure is etched to expose the n-GaN pillar second end and connected to a second metal layer.

RELATED APPLICATIONS

This application is a Continuation-in-Part of a pending applicationentitled, METHOD FOR FABRICATING THREE-DIMENSIONAL GALLIUM NITRIDESTRUCTURES WITH PLANAR SURFACES, invented by M. Albert Crowder et al.,Ser. No. 13/337,843, filed Dec. 27, 2011, which is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to light emitting diode (LED)fabrication processes and, more particularly, to a method forfabricating three-dimensional (3D) gallium nitride structures withplanar surfaces for use in LEDs.

2. Description of the Related Art

FIG. 1 is a partial cross-sectional view of a planar gallium nitride LED(prior art). Gallium nitride (GaN) is widely used for LED applicationsdue to its favorable band-gap and direct band structure, and mostfabrication follows a planar metalorganic chemical vapor deposition(MOCVD) sequence, as noted by Nguyen, X. L., Nguyen, T. N. N., Chau, V.T. & Dang, M. C., “The fabrication of GaN-based light emitting diodes(LEDs)”, Adv. Nat. Sci: Nanosci. Nanotechnol. 1, 025015 (2010), asfollows:

1) A thick n-GaN with Si doping is deposited on a sapphire substrate;

2) A multiple quantum well (MQW) layer is formed consisting ofalternating thin layers of InGaN and AlGaN; and,

3) A thin p-GaN layer is formed with Mg doping.

One of the constraints on this technology is the high cost of producingGaN for devices due to the difficulties encountered in forming asufficient high-quality material. These difficulties primarily stem fromthe growth process, which is typically conducted at very hightemperatures (e.g., over 1,000° C.) in molecular beam epitaxy (MBE) orMOCVD reactors and on substrates with a different coefficient of thermalexpansion (CTE). The difference in CTE can lead to formation ofthreading dislocations that adversely affect device performance andreliability. In addition, film stress limits the amount of dopants thatcan be incorporated in a GaN film, which in turn limits the range ofemission characteristics that are achievable. So it would be desirableto improve the defect density and increase the amount of device surfacearea that can produce LED emission for a given area of growth substrate.

FIGS. 2A through 2C are partial cross-sectional views of LED deviceswith textured surfaces (prior art). FIG. 2A depicts a planar LED, FIG.2B depicts a flip chip LED, and FIG. 2C depicts a textured template LED.One other consideration of the planar LED structure is the high index ofrefraction of GaN, which limits the amount of light that can be emittedto a narrow angular cone. Light outside the escape cone is reflectedinternally, diminishing the efficiency of the device. A variety of wayshave been devised to enhance the roughness of the encapsulation layer onplanar devices as a means to allow more light to escape, see Fujii, T.et al., “Increase in the extraction efficiency of GaN-basedlight-emitting diodes via surface roughening”, Applied Physics Letters84, 855 (2004), and the dry etch texturing study of Lee, H. C. et al.,“Effect of the surface texturing shapes fabricated using dry etching onthe extraction efficiency of vertical light-emitting diodes”,Solid-State Electronics 52, 1193-1196 (2008). Nanostructured surfacecoatings have also been used in a similar way to extract internalreflections (Kang, J. W. et al., “Improved Light Extraction of GaN-BasedGreen Light-Emitting Diodes with an Antireflection Layer of ZnO NanorodArrays”, Electrochem. Solid-State Lett. 14, H120-H123 (2011).

FIGS. 3A and 3B depict, respectively, a GaN micro-rod LED structure anda device fabricated from an array of micro-rod LEDs (prior art). Onemethod to alleviate the problems with planar device fabrication is touse GaN nanowires or micro-rods (μ-rods). Such structures can befabricated at high temperature with the appropriate shell structures toform a p-QW-n LED, harvested from the growth substrate, and depositedusing a dielectrophoresis (i.e., e-field) process. GaN μ-rods provide anon-planar template, often in the form of a hexagonal or triangular rod,for the epitaxial growth of quantum well (QW) structures. The divergencefrom planar should provide a higher efficiency for light extraction. Thediameter of the μ-rods and nanowires is typically small enough that thethreading dislocation density is significantly reduced, increasing theinternal quantum efficiency (IQE) and lifetime. By controlling thecrystallographic orientation of the GaN μ-rods, non-polar or semi-polarplanes can be used for device fabrication, thereby reducing the effectof the quantum confined stark effect (QCSE), which, in turn, can alsolead to improvements in the IQE.

Several research groups have worked on the development of GaN nanowiresto varying degrees of success. One approach that yields high-quality GaNnanowires was developed by UNM researchers, and uses MOCVD epitaxialgrowth from a templated substrate (S. D. Hersee, et al., “The controlledgrowth of GaN nanowires”, Nano Letters 6, 1808 (2006). This processyielded good nanowires with a constant diameter and a hexagonalcross-section with sidewall orientations in the (1100) family. However,the growth was limited to 2 μm per hour.

Other VLS-based growth processes have been developed using variouscatalysts (e.g., Ge, Au, or Fe), with resulting nanowires and nanorodsbeing produced and fabricated into LED devices. The crystallographicorientation of VLS-grown GaN nanowires can be non-ideal, as there arecompeting preferred axial orientations for growth (a- and c-axis,depending on temperature), competing phases (zinc-blend and wurtzite),and the resulting nanowires can have non-uniform sidewall orientations.This can affect the uniformity of e-field dispersed GaN nanowires thatare used for device fabrication.

It would be advantageous if a GaN LED could be fabricated with uniformsidewall orientations and a minimal density of defects.

SUMMARY OF THE INVENTION

Disclosed herein is a class of structures for gallium nitride (GaN)based light emitting diodes (LEDs) that have improved performancebecause of the 3-dimensional shape of the initial GaN template overwhich the LED layers are deposited. The template shape can be columnar,which forms micro-rod or pillar LEDs, and can be connected in an arrayto make high efficiency lighting of arbitrary size and shape.Alternatively, the template can have a series of pits that increase theemission area and improve light extraction for a planar device. Templateshapes are formed by a combination of a damage etch and a wet etch thatselectively removes damaged GaN, leaving high quality low etch ratecrystalline planes. LED devices made from these template shapes areunlike conventional technologies that fabricate LED structures by MOCVDdeposition on planar n-type GaN substrates, creating large planardevices. Further, unlike conventional devices, LED devices made from theabove-mentioned templates do not require addition top surface texturingafter the LED device is formed.

Accordingly, a method is provided for fabricating a LED usingthree-dimensional GaN pillar structures with planar surfaces. The methodforms a plurality of GaN pillar structures. Each GaN pillar structure isa result of forming an n-doped GaN (n-GaN) pillar having a first end, asecond end, with at least one of the ends formed in a c-plane, andplanar sidewalls perpendicular to the c-plane, formed in either anm-plane or a-plane family. A multiple quantum well (MQW) layer is formedoverlying the n-GaN pillar sidewalls, and a layer of p-doped GaN (p-GaN)is formed overlying the MQW layer. The plurality of GaN pillarstructures are deposited on a first substrate, with the n-doped GaNpillar sidewalls aligned parallel to a top surface of the firstsubstrate. A first end of each GaN pillar structure is connected to afirst metal layer to form a first electrode. The second end of each GaNpillar structure is etched to expose the n-GaN pillar second end andconnected to a second metal layer to form a second electrode.

In another aspect, the method grows an n-GaN film overlying a substrate.A plurality of openings is formed in a first region of the n-GaN film.Each opening has planar sidewalls perpendicular to a c-plane alignedwith a top surface of the n-GaN film, and formed in either an m-plane ora-plane family. A MQW layer is formed overlying the first region ofn-GaN film, and a layer of p-GaN is formed overlying the MQW layer. Afirst metal layer is deposited overlying a second region of the n-GaNfilm forming a first electrode. A second metal layer is depositedoverlying the p-GaN film to form a second electrode.

Additional details of the above-mentioned methods, and LEDs withthree-dimensional GaN pillar structures having planar surfaces, areprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a planar gallium nitride LED(prior art).

FIGS. 2A through 2C are partial cross-sectional views of LED deviceswith textured surfaces (prior art).

FIGS. 3A and 3B depict, respectively, a GaN micro-rod LED structure anda device fabricated from an array of micro-rod LEDs (prior art).

FIGS. 4A and 4B are partial cross-sectional views of a GaNthree-dimensional (3D) structure.

FIG. 5 is a partial cross-section view depicting a GaN 3D array.

FIGS. 6A and 6B are plan views of the array of FIG. 5.

FIG. 7 is a perspective view showing a GaN micro-pillar array etchedusing Cl₂-based plasma dry etch.

FIG. 8 depicts schematic diagrams of the cross-sectional GaN film viewedalong the direction for N-polar GaN to explain the mechanism of thepolarity selective etching.

FIG. 9 depicts hexagonal-shaped etch pits due to the selective attack ofthe etchant on the dislocation defects spots.

FIGS. 10A through 10C depict selective GaN etches with defectsintroduced by a plasma dry etch in TMAH etchant.

FIG. 11 depicts hexagonal-shaped GaN micro posts, which are a byproductof a circular micro post etch.

FIGS. 12A and 12B depict possible etch configurations for the combinedlaser drilling plus anisotropic wet etch process.

FIGS. 13A through 13G depict steps in an exemplary process fabricating aplanar honeycomb LED structure enhanced by three dimensional texturing.

FIGS. 14A through 14O depict steps in an exemplary process forfabricating a micro-rod or pillar LED device.

FIG. 15 is a flowchart illustrating an alternative method forfabricating a LED using three-dimensional GaN pillar structures withplanar surfaces.

FIGS. 16A and 16B are scanning electron microscope (SEM) imagerepresentations of, respectively, a honeycomb structure and triangularetched pillars.

FIGS. 17A and 17B are, respectively, plan and partial cross-sectionalviews of another type of GaN 3D array.

FIGS. 18A and 18B are, respectively, plan and partial cross-sectionalviews of a LED with three-dimensional GaN pillar structures havingplanar surfaces.

FIGS. 19A and 19B are, respectively, plan and cross-sectional views of arelated type of LED with three-dimensional GaN pillar structures withplanar surfaces.

FIGS. 20A and 20B are flowcharts illustrating a method for fabricating aLED using three-dimensional GaN pillar structures with planar surfaces.

DETAILED DESCRIPTION

FIGS. 4A and 4B are partial cross-sectional views of a GaNthree-dimensional (3D) structure. The GaN 3D structure 400 comprises aGaN pillar 402 and pillar sidewalls 404. The pillar sidewalls 404 areformed in either the m-plane (10 10) or a-plane ( 12 10). Each family ofplanes (m and a) both contain 6 faces. The 6 faces for the m-planefamily are {10 10}, { 1010}, {01 10}, {0 110}, {1 100}, and { 1100}. Forthe a-plane family, the faces are {1 210}, { 12 10}, {11 20}, {2 1 10},{ 1 120}, and { 2110}. Note that for these Miller indices, families arein (parentheses), faces are in {curly brackets}, and zone axes are in[square brackets]. As shown in FIG. 4B, the GaN pillar 402 has atriangular pattern. However, hexagonally shaped pillars mayalternatively be formed, see FIG. 6B.

FIG. 5 is a partial cross-section view depicting a GaN 3D array. Thearray 500 comprises a substrate 502 with a top surface 504, with anarray of GaN structures 506. Each GaN structure 506 has a first end 508attached to the substrate top surface 504, and sidewalls 510 formed inthe m-plane or a-plane. Typically, the substrate 502 is a material suchas sapphire, silicon, or silicon carbide. However, the array 500 is notlimited to any particular substrate material.

FIGS. 6A and 6B are plan views of the array of FIG. 5. The GaNstructures 506 of FIG. 5 are GaN pillars. As shown in FIG. 6A, the GaNpillars or micro-rods 402 may have a triangular shape. As shown in FIG.6B, the GaN pillars having a hexagonal shape.

FIGS. 16A and 16B are, respectively, plan and partial cross-sectionalviews of another type of GaN 3D array. The array 1700 comprises asubstrate 1702 with a top surface 1704. A honeycomb structure ofopenings 1706 are formed in a GaN film 1708. Each opening 1706 hassidewalls 1710 formed in the m-plane or a-plane. Typically, the openings1706 formed in the GaN film 1708 have a hexagonal shape. The substrate1702 may be sapphire, silicon, or silicon carbide.

FIGS. 18A and 18B are, respectively, plan and partial cross-sectionalviews of a LED with three-dimensional GaN pillar structures havingplanar surfaces. The LED 1800 comprises a substrate 1802 with a topsurface 1804 having a plurality of first electrode digits (fingers) 1806formed in a first metal layer, and a plurality of opposing secondelectrode digits 1808 formed in a second metal layer. A plurality of GaNpillar structures 1810 is shown. Each GaN pillar structure 1810 bridgesa gap between a first electrode digit 1806 and a corresponding secondelectrode digit 1808. Each GaN pillar structure 1810 comprises ann-doped GaN (n-GaN) pillar 1812 having a first end 1814 overlying acorresponding first electrode digit 1806, a second end 1816 connected toa corresponding second electrode digit 1808. At least one of the ends isformed in a c-plane. In one aspect, the second end 1816 is typicallyformed in the c-plane. The n-GaN pillars 1812 have planar sidewalls 1818perpendicular to the c-plane, formed in either an m-plane and a-planefamily. A multiple quantum well (MQW) layer 1820 overlies the n-GaNpillar sidewalls 1818 and the GaN pillar first end 1814. A layer ofp-doped GaN (p-GaN) 1822 overlies the MQW layer 1820. As describedabove, the n-doped GaN pillars 1812 have either a hexagonal ortriangular shape.

Optionally as shown, a thin Ni layer 1824 overlies the p-GaN layer 1822of each GaN pillar structure 1810. A transparent conductor 1826 overliesthe Ni layer 1824, so that the combination of the Ni layer 1824 andtransparent conductor 1826 electrically connect the p-GaN layer 1822 ofeach GaN pillar structure 1810 to a corresponding first electrode digit1806. A metal interconnect 1828 is interposed between each exposed n-GaNpillar second end 1816 and the underlying second electrode digit 1808.

FIGS. 19A and 19B are, respectively, plan and cross-sectional views of arelated type of LED with three-dimensional GaN pillar structures withplanar surfaces. The LED 1900 comprises an n-GaN film overlying asubstrate 1902. The n-GaN film has a first region 1904 a with aplurality of openings 1906 and a second region 1904 b. Typically, theopenings 1906 have a hexagonal shape. Each opening 1906 has planarsidewalls 1908 perpendicular to a c-plane aligned with a top surface1910 of the n-GaN film 1904 a. The planar sidewalls 1908 are formed ineither an m-plane or a-plane family. A MQW layer 1910 overlies the n-GaNfilm first region 1904 a. A layer of p-GaN 1912 overlies the MQW layer1910. A first metal layer 1914 overlies the second region of n-GaN film1904 b forming a first electrode. A second metal layer 1916 overlies thep-GaN film 1912 to form a second electrode. In another aspect, the LEDstructure of FIGS. 19A and 19B may include an optional current spreadinglayer consisting of a Ni layer and a transparent conductor, as describedin more detail below.

Planar GaN films deposited on sapphire substrates can be used to formetched 3-dimensional structures such as rods or cavities that can act asa template for subsequent epitaxial growth of doped GaN layers to makeLED devices with unconventional topologies. The general method offabrication uses a technique such as plasma etch or laser ablation toform a damage region in the GaN layer, followed by a wet etch thatselectively removes the damaged material. The final shape is formed bycrystallographic planes of GaN that have very low etch rates and havedesirable properties for device fabrication such as low density ofdislocations and trap states. One aspect of the first etch step is toremove material and create a damage profile with a shape such as amicro-rod or conical cavity that is appropriate for the type of deviceto be made. Other techniques such as ion implant or sand blasting mayalso be used to form the damage region. The wet etch step refines theinitial shape and produces surfaces with low damage that can make highquality devices.

One aspect forms columnar templates and deposits the layers required tomake an LED over the template to make micro-rod LEDs that can beharvested, deposited on a new substrate, and wired together to make anarray of LED emitters.

One other consideration of the planar LED structure is the high index ofrefraction of GaN, which limits the amount of light that can be emittedto a narrow angular cone. Light outside the escape cone is reflectedinternally, diminishing the efficiency of the device. As noted above, avariety of ways have been devised to enhance the roughness of theencapsulation layer of conventional planar devices as a means to allowmore light to escape. The structures disclosed herein improve lightextraction by etching a surface texture in the n-type GaN startingmaterial before LED fabrication, which also increases the emission areacompared to a conventional planar LED.

To fabricate high performance of GaN based devices, GaN patterning (i.e.etching) techniques are crucial. Variations in the quality of theas-grown GaN, coupled with the high bond energies associated with “III-nitride” materials, present unique challenges for etching processes.Similar to etching other semiconductor materials, plasma based dry etchand chemical based wet etch are the two major etch techniques for GaNpatterning. Laser patterning GaN film is also capable of achievingunique structures when combined with proper wet etching techniques toremove the ablated material and the thermal decomposition.

Typical etching gases for GaN plasma dry etch are Cl₂/Ar. Argon (orhelium) is added to stabilize the plasma or for cooling purposes. Argonaddition causes inert ion bombardment of the surface, which results inenhanced anisotropic etching, while the chlorine-based plasma produces(volatile) chemical byproducts such as GaCl₃. The dry etch process canachieve a highly anisotropic etch with a high etching rate and has asmooth surface morphology. Using Cl₂-based plasma to etch GaN isdesirable because chlorine-based gas chemistry is widely used in theprocessing of semiconductor devices.

FIG. 7 is a perspective view showing a GaN micro-pillar array etchedusing Cl₂-based plasma dry etch. The etch parameters may be controlled.Defects in the GaN appear to be particularly sensitive to etchingconditions and respond by etching faster or slower than the surroundingmaterial, ultimately forming pits or grass, as shown.

One facet of GaN plasma dry etching is that it is easy to generateion-induce damages, which may degrade the GaN based devices performance.To address this issue, the dry and wet etching techniques may becombined, or laser ablation and wet etching techniques may be combined.

As mentioned above, most conventional processes deposit GaN on foreignsubstrates, such as sapphire or silicon carbide. Wet etching studieshave previously been limited almost exclusively to the (0001)orientation, since until recently, only such epitaxial films wereavailable. A variety of chemistries have been demonstrated as being ableto attack specific planes in the GaN crystallographic system, as shownin Table 1.

TABLE I Etch rates and observed etching planes for various chemicals.Temperature Etch rate Etching planes Chemical (° C.) (μm/min) observedAcetic acid (CH₃COOH) 30 <0.001 None Hydrochloric acid (HCl) 50 <0.001None Nitric acid (HNO₃) 81 <0.001 None Phosphoric acid (H₃PO₄) 108-1950.013-3.2 {10 1 2}, {10 13} Sulphuric acid (H₂SO₄) 93 <0.001 NonePotassium hydroxide 150-247 0.003-2.3 {10 10}, {10 1 1} (KOH), molten50% KOH in H₂O 83 <0.001 None 10%-50% KOH in  90-182 0.0015-1.3  {10 10}ethylene glycol (CH₂OHCH₂OH) 50% NaOH in H₂O 100  <0.001 None 20% NaOHin ethylene 178   0.67-1.0 None glycol

GaN can be etched in, an aqueous base solution, however, etching ceasesupon the formation of an insoluble coating of presumably galliumhydroxide (Ga(OH)₃). For (0001) orientation GaN films, there are twotypes surfaces polarity, one is Ga-polar GaN and another is N-polar GaN.Most GaN films grown by metal organic chemical vapor deposition (MOCVD)or hydride vapor phase epitaxy (HVPE) are Ga-polar films, and GaN filmsgrown by metal organic molecular beam epitaxy (MOMBE) are N-polar films.As noted in “Crystallographic wet chemical etching of GaN” APL v. 73. n.18, 1998, p. 2655, in aqueous KOH, NaOH or TMAH, only nitrogen polar GaNfilms were etched, and produced triangular shaped pyramids limited by(11 21) planes. No etching of Ga-polar films occurred in aqueous KOH,NaOH or TMAH.

The different etching characteristics of Ga-polar and N-polar crystalsare due to the different states of surface bonding and are onlydependent on polarities. The mechanism of such polarity selectiveetching was interpreted by D. Li, M. Sumiya, K. Yoshimura, Y. Suzuki, Y.Fukuda, S. Fuke, Phys. Status Solidi A 180 (2000) 357.

FIG. 8 depicts schematic diagrams of the cross-sectional GaN film viewedalong the direction for N-polar GaN to explain the mechanism of thepolarity selective etching. Stage (a) shows a nitrogen terminated layerwith one negatively charged dangling bond on each nitrogen atom. Stage(b) depicts the adsorption of hydroxide ions. Stage (c) shows theformation of oxides. Stage (d) depicts dissolving of the oxides.

The hydroxide ions (OH—) are first adsorbed on the sample surface andsubsequently react with Ga atoms following the reaction:

KOH works as a catalyst and is also a solvent for the resulting Ga₂O₃(Step (d)). As the stages of (a) to (d) in FIG. 8 repeat, the N-polarGaN can be etched. Note that it does not matter which atoms form thesurface termination layer. If the surface is Ga-terminated, the etchingcan be initialized by Stage (c). In contrast, the inertness of Ga-polarGaN is ascribed to the repulsion between (OH—) and three occupieddangling bonds of nitrogen, which prevents the hydroxide ions fromattacking the Ga atoms, so that the Ga-polar GaN films are not etched.

However, the large lattice mismatch between the epitaxy layer andsubstrate results in a high dislocation density in GaN, typically in therange of 10⁷-10¹¹ cm². Such defects have a pronounced effect on theetching rate and the resulting surface morphology.

FIG. 9 depicts hexagonal-shaped etch pits due to the selective attack ofthe etchant on the dislocation defects spots. As noted by Seok-In Na etal., “Selective Wet Etching of p-GaN for Efficient GaN-BasedLight-Emitting Diodes”, IEEE Photonics Technology Letters, Vol. 18, No.14, Jul. 15, 2006, not only can the dislocation-related selective etchhappen in the base etchant, defects introduced by plasma dry etch canalso initiate the same kind of selective etch, as shown.

FIGS. 10A through 10C depict selective GaN etches with defectsintroduced by a plasma dry etch in TMAH etchant. FIG. 10A shows the GaNsurface after a 1-minute plasma etch. FIG. 10B shows the same surfaceafter a TMAH wet etch. FIG. 10C is a close-up view of a hexagonal pit.Thus, the etching is done in two steps: first a circular cavity isplasma dry etched into the (0001) planes (FIG. 10A), thencrystallographic etch delineates the slow etch rate planes (FIG. 10B).In this case, the plane families (10 10) are perpendicular to thec-plane in which the hexagonal pits are formed. The (10 10) planefamilies can not only form the inner sidewalls of hexagonal pits, theycan also be the sidewall of triangle shape GaN micro posts outersidewall, as shown in FIG. 11.

FIG. 11 depicts hexagonal-shaped GaN micro posts, which are a byproductof a circular micro post etch. Clearly, the complementary mask for dryetching micro posts in FIG. 11, followed by the TMAH crystallographicetch, yields full height hexagonal-shaped GaN micro posts or pillars.There are many advantages to fabricating hexagonal-shaped GaN microposts, as discussed below. It should be understood that the introductionof the initial defects need not be limited to a plasma dry etch. Thelaser ablation of GaN followed by crystallographic etch in TMAH createsa similar result. Other techniques used to generate damage regions in aregular pattern such as ion implantation may also work.

For controlled defects generation by laser ablation, the planar GaN filmis subjected to pulsed excimer laser irradiation in order to inducethermal decomposition of the GaN into metallic gallium and nitrogen. Thenoncoherent nature of the excimer laser permits the irradiation ofmultiple regions simultaneously, although coherent light sources mayalso be used with diffractive optics.

One pattern consists of a hexagonal array of dots that effectivelydrills holes into the GaN film to a predetermined penetration depth.This depth is controllable by the number of pulses that are allowed toimpinge on a given area and by the energy density of the laser pulse.This laser drilling induces defects in the sidewalls of the affectedregion which can be subsequently etched anisotropically with a wet etchsuch as heated dilute TMAH (tetramethyl ammonium hydroxide). Thisanisotropic etch removes the defective material and effectively stops onthe c-plane and m-planes in the film, resulting in a hexagonal etch pitthat extends from the GaN film surface to the laser penetration depth.By manipulating the angular orientation of the hexagonal dot array thatis used for irradiating the surface with the crystallographicorientation of the GaN film, a predetermined pattern can be formed inthe final laser-drilled and wet-etched GaN. For example, if the laserarray is aligned to the m-plane orientation, the laser-drilled pits canbe etched to end up with a honeycomb structure, as the slowest-etchedfacets are perpendicular to the nearest neighbor pit. Conversely, whenthe laser array is aligned to the a-plane orientation (i.e., rotatedfrom the first orientation by 30°), the corners of the etched hexagonalpits impinge upon those from their nearest neighbors, and triangularstructures can form if the wet etch process proceeds for a sufficientamount of time.

FIGS. 12A and 12B depict possible etch configurations for the combinedlaser drilling plus anisotropic wet etch process. The laser pattern inFIG. 12A is aligned to the m-plane GaN orientations, while the laserpattern in FIG. 12B is aligned to the a-plane GaN orientation. The blackareas represent the initial laser drilling hole, while the gray areasrepresent the GaN removed by the anisotropic wet etch.

The first of the two configurations (FIG. 12A) results in a well-orderedarray of etched pits that have only the c-planes and m-planes exposed.This increase in surface area can be exploited for planar LEDapplications, allowing epitaxial growth of GaInN and p-GaN over thebest-suited crystallographic planes. Due to the nature of the laserprocess, the depth and sidewall slope of the etched pits can beprecisely controlled by the laser fluence and shot count. A typicalexample for forming etch pits in the GaN would involve 100 shots perarea with a 308 nm laser fluence of 1.7 J/cm². This is then etched in apiranha bath (H₂O₂ and H₂SO₄ at 140° C. for 20 minutes) to remove theejected Ga metal, and then etched for 4 to 120 hours in 75° C. TMAH(5%).

The second of the two configurations (FIG. 12B) is well-suited forforming etched vertical μ-rods or pillars. Carrying out the anisotropicetch until the corners of the hexagonal pits impinge upon one anotherallows the bulk of the GaN material to be removed, leaving onlytriangular vertical pillars. The anisotropic etch leaves behind them-planes (the family of (1 100) planes in the wurtzite structure, unlessthey are the a-planes, which are (1 120) families), yielding atriangular pillar with all faces in the same family of planes. Them-planes are non-polar, making them more favorable for LED deviceapplications.

Experiments show that there is a sharp threshold fluence atapproximately 1,100 mJ/cm² for the laser process to induce thermaldecomposition of the GaN films, that is independent of the number ofpulses irradiated onto a specific area. The uniformity of laser-induceddamage at or close to this threshold is poor due to the stochasticnature of the thermal decomposition and variations in the spatialprofile of the laser pulse. However, at higher fluences where thermaldecomposition is more readily and uniformly achieved, the number oflaser pulses can be seen to have a secondary effect on the sidewallprofile and depth of the damaged region in the GaN film. This isparticularly noteworthy where a lower shot count (i.e., 30 shots perarea) results in shallower etch depth with a more pronounced tapering intowards the center of the etched pit. At higher shot counts per area,the sidewall profile is steeper, and there is less tapering in towardsthe center with increasing penetration. This is a result of the morenumerous laser pulses being responsible for ejecting the molten metallicgallium away from the drilled hole, thereby allowing the thermaldecomposition of material at the bottom of the laser-drilled hole toextend laterally from the center.

Experiments were also conducted to observe the effect of anisotropicetching in 5% TMAH at 85° C. for 138 hours following damage inducementby laser drilling. The laser drilling causes extensive damage to the GaNfilm through the thermal decomposition of the GaN, as well as by theappearance of steep thermal gradients in the localized regionsurrounding the laser-drilled holes. This damaged material isanisotropically etched by the heated dilute TMAH, which readily attacksthe damaged GaN, but is slowed by certain crystallographic places, suchas the c-planes and m-planes. This ability to control the profile of thedamaged region relative to the crystallographic layout of the GaN film,either by dry etching or by laser drilling, permits control over thefinal shape of the 3D template being produced.

The laser drilling process is an effective means of inducing damage inthe GaN film, especially with a projection-based, excimer-laser-basedprocessing tool. The projection system coupled with a high-power excimerlaser allows for a large region to be exposed simultaneously with auniform fluence that is sufficient to cause thermal decomposition of theGaN. This type of system also has a wide depth of field, typicallygreater than 25 μm, which eliminates the problems encountered withsubstrate bowing due to the CTE mismatch during growth of the GaN. Thebowing of the substrates is approximately 7-10 μm, which is difficult toovercome with photolithography as used for dry etching of the GaN toform the etched pits prior to etching in TMAH. Excimer lasers are pulsedlaser systems that can operate at relatively high frequencies (typically300 Hz, although some laser systems can go up to 4 kHz), allowing highthroughput processing of GaN on sapphire substrates.

FIGS. 13A through 13G depict steps in an exemplary process fabricating aplanar honeycomb LED structure enhanced by three dimensional texturing.FIG. 13A begins with a sapphire substrate and grows a thick n-GaN layerusing a MOCVD or MBE process. In FIG. 13B, deep cone-shaped pits areetched or ablated in the n-GaN. In FIG. 13C a wet etch is used, asdescribed above, to remove the damaged GaN. In FIG. 13D a thin n-GaNlayer may optionally deposited. Then, the MQW and p-GAN layers aredeposited to form an LED on the n-GaN template. In FIG. 13E an openingis etched to contact then-GaN. A metal is deposited on p-GaN and n-GaNcontacts using appropriate metal appropriate to prevent a Schottkybarrier. In FIG. 13F the sapphire wafer is sawed to singulate devicesand an array of devices is attached to a substrate. In FIG. 13Gconnections are made to the LED with wire bonding. In one aspect, a thinlayer of Ni is conformally deposited to make ohmic contact with thep-GaN layer, and a transparent conductor such as ITO, ZnO, carbonnanotubes (CNTs), or graphene is conformally deposited as a currentspreading layer, prior to forming the metal contact.

FIGS. 14A through 14O depict steps in an exemplary process forfabricating a micro-rod or pillar LED device. FIG. 14A begins with asapphire substrate and an n-GaN layer is grown by MOCVD or MBE,typically 10 to 30 microns (μm) thick. In FIG. 14B the n-GaN is etchedor ablated to form either triangular or hexagonal pillars. In FIG. 14C awet etch is used, as described above, to remove the damaged GaN formingvertical pillars. In FIG. 14D a thin n-GaN layer is optionally formed.MQW and p-GAN layers are deposited to form an LED on the n-GaN template.The MQW layer is a series of quantum well shells (typically 5layers—alternating 5 nm of InGaN with 9 nm of n-GaN. There may also bean AlGaN electron blocking layer (not shown) between MQW layers andp-GaN. The outer shell is p-doped GaN (Mg doping) about 200 nm thick. Ahigh-brightness blue LED can be formed, or a green LED if a higherindium content is used in the MQW. In FIG. 14E the micro-rods (pillars)are harvested from the sapphire wafer using laser lift-off. For example,the substrate and n-GaN layer may be attached to a Si handling waferusing a thermoplastic polymer. In one aspect the pillars are detachedusing a 1-shot XeCl laser (λ=304 nm) at 1100 mJ/cm². After detachment,the thermoplastic polymer can be dissolved, in acetone to separate thehandling wafer.

In summary, the above-described fabrication processes are a combinationof a damage-inducing etch of GaN films, with an anisotropiccrystallographic wet etch, to produce predetermined 3D structures. Usingthese methods, etch pits in GaN can form an array of hexagonal taperedpits bounded by the crystallographic m and c planes. Triangular,vertical GaN μ-rods with m-family {1 100} sidewalls can be made that areideal (i.e., non-polar) templates for LED applications. Hexagonal,vertical GaN μ-rods with a-family {11 20} sidewalls can be made that areideal (i.e., non-polar) templates for LED applications.

In FIG. 14E the LED structure fabrication begins with a planar substratethat can be glass or plastic. For example, a Gen 6 LCD glass substrate(1500×1850 mm) can be used. A metal layer is deposited, such as Ta or Alfor example, and patterned to make e-field electrodes. Positive andnegative bus lines connect multiple pairs of e-field fingers. Fingers(digits) are positioned where GaN pillar structures are to be deposited.Typically, the space between fingers is less than the pillar structurelength.

In FIG. 14F a suitable solvent (e.g., isopropyl alcohol, water, oracetone) is flowed over the substrate surface. A modulated AC field isapplied to the e-field structure to produce a capture field between thefingers.

In FIG. 14G, after the surface is completely wetted, ink with suspendedGaN pillar structures is injected. The ink flow and field are adjustedto capture pillar structures between e-field electrode fingers. At ahigh flow and low field, GaN pillar structures are not captured. At lowflow and high field strength, multiple GaN pillar structures may becaptured. When the flow and field are balanced, only one GaN pillarstructure is captured per position, which is the objective.

In FIG. 14H, when there are GaN pillars captured at each position, thefield is increased to pin the GaN pillars to the substrate. Solvent isflowed instead of ink, and the flow rate is increased to flush outexcess non-captured GaN pillar structures, leaving one GaN pillarstructure centered at each position.

In FIG. 14I, when the solvent has dried, the structure is ready fordevice fabrication. Because the p-GaN layer has very low conductivity, acurrent spreading layer may be used. A very thin layer of Ni (typically2 nm thick) is conformally deposited to make ohmic contact with theouter p-GaN shell layer. A transparent conductor such as ITO, ZnO,carbon nanotubes (CNTs), or graphene is conformally deposited as acurrent spreading layer.

In FIG. 14IJ the current spreading layer is patterned and etched,leaving room on one end of the wire to make contact to the n-GaN pillar.A wet etch may be used to pattern most conductive transparent oxides. Anoxygen plasma etch may be used for CNT or graphene.

In FIG. 14K, one end of the GaN pillar structure is patterned and etchedto make electrical contact with the n-GaN pillar core. Using a Cl₂ basedreactive ion etch (RIE), the p-GaN shell and the multiple quantum welllayers can be removed to reveal the n-GaN core. As shown, there may besome over-etch into the core.

In FIG. 14L a metal interconnect is deposited to contact the n-GaNpillar core and make the negative voltage bus lines. The metal may, forexample, be Au or Al with a Ti interface layer to make ohmic contactwith n-GaN.

In FIG. 14M a metal interconnect is deposited to contact the currentspreading layer. The metal may be Au or Al with a Ni interface layer.Alternatively, this metal could be deposited first and the n-GaN metaldeposited over it. As another alternative, metal can also be depositedfor both n-GaN and current spreading layer in the same step.

FIG. 14N depicts a single cell with parallel connected GaN pillarstructures. The cell is conceptually simple. The amount of lightproduced is proportional to the number of GaN pillar structures. Forexample, an operating of about 3.4 V may be used.

FIG. 14O depicts a cell with series-parallel connections with threediode strings in series, which may use an operating voltage up to about10 V. A more practical product is likely to be bigger withparallel-series connections having hundreds or thousands of GaN pillarstructures, depending on the requirements.

FIGS. 16A and 16B are scanning electron microscope (SEM) imagerepresentations of, respectively, a honeycomb structure and triangularetched pillars. Both samples were processed with 120 laser shots perarea at 1.6 J/cm². The substrate flat orientation is typically alignedto either a-axis or m-axis. A 30° offset (e.g., m-axis flat orientationand no compensation in rotation for laser process) can lead to formationof a honeycomb structure, instead of triangular μ-rods.

By etching hexagonal arrays of laser drilled holes with heated 5% TMAH,triangular μ-rods with controlled crystallographic faces are formed. Thelaser process has a much wider depth of field than lithographicprocesses, bypassing the issues associated with wafer bowing fromthermal stress. The anisotropic TMAH etch is selective to the c-plane,as well as to the m-planes. This permits the formation of triangles withthe c-plane (0001) at the end, and m-planes (1100) on all three verticalsides.

A similar structure is possible through conventional VLS growth of GaNnanowires (Nano Let., v.6, n.8, 2006, p.1808), with the sides aligned tothe family of m-axes, and the top aligned to the c-axis. However, theseare reported to be hexagonal in shape and require very high temperatureMOCVD processes (1050° C.) for the growth. The hexagonal structure cancause reentrant regions that make some aspects of LED fabrication moredifficult.

FIG. 20 is a flowchart illustrating a method for fabricating a LED usingthree-dimensional GaN pillar structures with planar surfaces. Althoughthe method is depicted as a sequence of numbered steps for clarity, thenumbering does not necessarily dictate the order of the steps. It shouldbe understood that some of these steps may be skipped, performed inparallel, or performed without the requirement of maintaining a strictorder of sequence. Generally however, the method follows the numericorder of the depicted steps. The method starts at Step 2000.

Step 2002 forms a plurality of GaN pillar structures, each GaN pillarstructure being formed with the following sub-steps. Step 2002 a formsan n-GaN pillar having a first end, a second end, with at least one ofthe ends formed in a c-plane. The n-GaN pillar is formed with planarsidewalls perpendicular to the c-plane, formed in either an m-plane ora-plane family. The n-doped GaN pillars have a hexagonal or triangularshape. Step 2002 b forms a MQW layer overlying the n-GaN pillarsidewalls. Step 2002 c forms a layer of p-GaN overlying the MQW layer.Step 2004 deposits the plurality of GaN pillar structures on a firstsubstrate, with the n-doped GaN pillar sidewalls aligned parallel to atop surface of the first substrate. Step 2006 connects a first end ofeach GaN pillar structure to a first metal layer to form a firstelectrode. Step 2008 etches a second end of each GaN pillar structure toexpose the n-GaN pillar second end. Step 2010 connects the second end ofeach GaN pillar structure to a second metal layer to form a secondelectrode.

In one aspect, forming the n-doped GaN pillars in Step 2002 a includesthe following substeps. Step 2002 a 1 grows an n-doped GaN filmoverlying a second substrate. Step 2002 a 2 forms cavities in a topsurface of the GaN film. Step 2002 a 3 wet etches the cavities in theGaN film top surface. Step 2002 a 4 forms planar sidewalls extendinginto the GaN film that are perpendicular to a c-plane aligned with theGaN top surface. Step 2002 a 5 detaches the first end of each GaN pillarfrom the second substrate. In one aspect, forming the MQW layer (Step2002 b) and forming the p-GaN (Step 2002 c) are performed prior todetaching the first end of each GaN, pillar from the second substrate inStep 2002 a 5.

In one aspect Step 2003 provides the first substrate top surface with aplurality of first electrode digits and a plurality of opposing secondelectrode digits. Then, depositing the plurality of GaN pillarstructures on the first substrate in Step 2004 includes the followingsub-steps. Step 2004 a suspends the GaN pillar structures in an inksolution. Step 2004 b flows the ink solution over the first substratetop surface. Step 2004 c creates an alternating current (AC) electricfield with a first field strength between each first electrode digit andcorresponding second electrode digit. In response to the electric field,Step 2004 d bridges a gap between each first electrode digit andcorresponding second electrode digit with a GaN pillar structure.Subsequent to bridging the gap between first and second electrode digitswith the GaN pillar structure, Step 2004 e increases the electric fieldstrength to capture the GaN pillar structures. Step 2004 f flows asolvent over the first substrate top surface. In response to the solventflow, Step 2004 g removes GaN pillar structures not captured by theelectric field.

In one aspect, connecting the first end of each GaN pillar structure toa first metal layer in Step 2006 includes the following substeps. Step2006 a conformally deposits a Ni layer overlying the first substrate topsurface and GaN pillar structures. Step 2006 b conformally deposits atransparent conductor overlying the Ni layer. Then, prior to etching thesecond end of each GaN pillar structure, Step 2008 etches to remove thetransparent conductor and Ni layers overlying the second end of each GaNpillar structure. Step 2010 connects the second end of each GaN pillarstructure to the second metal layer by depositing a metal interconnectlayer overlying the exposed n-GaN to connect to the second metal layer.

FIG. 15 is a flowchart illustrating an alternative method forfabricating a LED using three-dimensional GaN pillar structures withplanar surfaces. The method starts at Step 2100. Step 2102 grows ann-GaN film overlying a substrate. Step 2104 forms a plurality ofopenings in a first region of the n-GaN film. Typically, the openingshave a hexagonal shape. Each opening has planar sidewalls perpendicularto a c-plane aligned with a top surface of the n-GaN film, and they areformed in either an m-plane or a-plane family. Step 2106 forms a MQWlayer overlying the first region of n-GaN film. Step 2108 forms a layerof p-GaN overlying the MQW layer. Step 2110 deposits a first metal layeroverlying a second region of the n-GaN film forming a first electrode.Step 2112 deposits a second metal layer overlying the p-GaN film to forma second electrode.

In one,aspect, forming the plurality of openings in the n-GaN film inStep 2104 includes substeps. Step 2104 a forms cavities in the topsurface of the n-GaN film. Step 2104 b wet etches the cavities in then-GaN film top surface. Step 2104 c forms planar sidewalls extendinginto the n-GaN film.

A LED made from GaN 3D planar structures and associated fabricationprocesses have been provided. Examples of particular process steps havebeen presented to illustrate the invention. However, the invention isnot limited to merely these examples. Other variations and embodimentsof the invention will occur to those skilled in the art.

We claim:
 1. A method for fabricating a light emitting diode (LED) usingthree-dimensional gallium nitride (GaN) pillar structures with planarsurfaces, the method comprising; forming a plurality of GaN pillarstructures, each GaN pillar structure being formed as follows: growingan n-doped GaN film overlying a second substrate; generating a patternof dislocation defects in a top surface of the GaN film; wet etching thedislocation defects in the GaN film top surface; removing GaN materialdamaged in response to forming the dislocation defects; stopping theremoval of GaN material in response to encountering c-planes, m-planes,and a-planes in the GaN film; forming an n-doped GaN (n-GaN) pillar corehaving a first end attached to the second substrate, a second end, withat least one of the ends formed in a c-plane, and planar sidewallsperpendicular to the c-plane, formed in a plane selected from a groupconsisting of an m-plane and a-plane family; forming a multiple quantumwell (MQW) shell surrounding the n-GaN pillar sidewalls; forming ap-doped GaN (p-GaN) shell surrounding the MQW shell; detaching aplurality of GaN pillar structures from the second substrate; depositingthe plurality of GaN pillar structures on a first substrate, with then-doped GaN pillar sidewalls aligned parallel to a top surface of thefirst substrate; connecting a first end of each GaN pillar structure toa first metal layer to form a first electrode; etching a second end ofeach GaN pillar structure to expose the n-GaN pillar second end; and,subsequent to etching the second end of each GaN pillar structure,connecting the second end of each GaN pillar structure to a second metallayer to form a second electrode.
 2. The method of claim 1 whereinforming the MQW shell surrounding the n-GaN pillar core sidewalls, andthe p-GaN shell surrounding the MQW shell includes forming the MQW shellsurrounding the n-GaN pillar core sidewalls, and the p-GaN shellsurrounding the MQW shell layer prior to detaching the first end of eachGaN pillar core from the second substrate.
 3. The method of claim 1wherein forming the n-doped GaN pillar cores includes forming pillarcores having a shape selected from a group consisting of hexagonal andtriangular.
 4. The method of claim 1 further comprising: providing thefirst substrate top surface with a plurality of first electrode digitsand a plurality of opposing second electrode digits; wherein depositingthe plurality of GaN pillar structures on the first substrate includes:suspending the GaN pillar structures in an ink solution; flowing the inksolution over the first substrate top surface; creating an alternatingcurrent (AC) electric field with a first field strength between eachfirst electrode digit and corresponding second electrode digit; and, inresponse to the electric field, bridging a gap between each firstelectrode digit and corresponding second electrode digit with a GaNpillar structure.
 5. The method of claim 4 wherein depositing theplurality of GaN pillar structures includes: subsequent to bridging thegap between first and second electrode digits with the GaN pillarstructure, increasing the electric field strength to capture the GaNpillar structures; flowing a solvent over the first substrate topsurface; and, in response to the solvent flow, removing GaN pillarstructures not captured by the electric field.
 6. The method of claim 5wherein connecting the first end of each GaN pillar structure to a firstmetal layer includes: conformally depositing a Ni layer overlying thefirst substrate top surface and GaN pillar structures; and, conformallydepositing a transparent conductor overlying the Ni layer.
 7. The methodof claim 6 wherein etching the second end of each GaN pillar structureto expose the n-GaN second end includes, prior to etching the second endof each GaN pillar structure, etching to remove the transparentconductor and Ni layers overlying the second end of each GaN pillarstructure; and, wherein connecting the second end of each GaN pillarstructure to the second metal layer includes forming a metalinterconnect layer interposed between the exposed n-GaN and the secondmetal layer.
 8. A method for fabricating a light emitting diode (LED)using three-dimensional gallium nitride (GaN) pillar structures withplanar surfaces, the method comprising: forming a plurality of GaNpillar structures as follows: growing an n-doped GaN film overlying asecond substrate; using a laser ablation process to induce thermaldecomposition in a top surface of the GaN film; in response to thethermal decomposition, forming cavities in the GaN film top surface; wetetching the cavities in the GaN film top surface; forming an n-doped GaN(n-GaN) pillar core having a first end attached to the second substrate,a second end, with at least one of the ends formed in a c-plane, andplanar sidewalls perpendicular to the c-plane, formed in a planeselected from a group consisting of an m-plane and a-plane family;forming a multiple quantum well (MQW) shell surrounding the n-GaN pillarsidewalls; forming a p-doped GaN (p-GaN) shell surrounding the MQWshell; detaching a plurality of GaN pillar structures from the secondsubstrate; depositing the plurality of GaN pillar structures on a firstbstrate, with the n-doped GaN pillar sidewalls aligned parallel to a topsurface of the first substrate; connecting a first end of each GaNpillar structure to a first metal layer to form a first electrode;etching a second end of each GaN pillar structure to expose the n-GaNpillar second end; and, subsequent to etching the second end of each GaNpillar structure, connecting the second end of each GaN pillar structureto a second metal layer to form a second electrode.
 9. The method ofclaim 8 wherein forming the MQW shell surrounding the n-GaN pillar coresidewalls, and the p-GaN shell surrounding the MQW shell includesforming the MQW shell surrounding the n-GaN pillar core sidewalls, andthe p-GaN shell surrounding the MQW shell layer prior to detaching thefirst end of each GaN pillar core from the second substrate.
 10. Themethod of claim 8 wherein forming the n-doped GaN pillar cores includesforming pillar cores having a shape selected from a group consisting ofhexagonal and triangular.
 11. The method of claim 8 further comprising;providing the first substrate top surface with a plurality of firstelectrode digits and a plurality of opposing second electrode digits;wherein depositing the plurality of GaN pillar structures on the firstsubstrate includes: suspending the GaN pillar structures in an inksolution; flowing the ink solution over the first substrate top surface;creating an alternating current (AC) electric field with a first fieldstrength between each first electrode digit and corresponding secondelectrode digit; and, in response to the electric field, bridging a gapbetween each first electrode digit and corresponding second electrodedigit with a GaN pillar structure,
 12. The method of claim 11 whereindepositing the plurality of GaN pillar structures includes: subsequentto bridging the gap between first and second electrode digits with theGaN pillar structure, increasing the electric field strength to capturethe GaN pillar structures; flowing a solvent over the first substratetop surface; and, in response to the solvent flow, removing GaN pillarstructures not captured by the electric field.
 13. The method of claim12 wherein connecting the first end of each GaN pillar structure to afirst metal layer includes: conformally depositing a Ni layer overlyingthe first substrate top surface and GaN pillar structures; and,conformally depositing a transparent conductor overlying the Ni layer.14. The method of claim 13 wherein etching the second end of each GaNpillar structure to expose the n-GaN second end includes, prior toetching the second end of each GaN pillar structure, etching to removethe transparent conductor and Ni layers overlying the second end of eachGaN pillar structure; and, wherein connecting the second end of each GaNpillar structure to the second metal layer includes forming a metalinterconnect layer interposed between the exposed n-GaN and the secondmetal layer.